The present disclosure is generally related to use of a spectral measuring device in connection with an outdoor accelerated weathering device, and more particularly, to methods for directly comparing the light spectral power distribution of a natural light source (the sun) to the light spectral power distribution received on the target area of an outdoor accelerated weathering device, comparing the spectral sensor on accelerated weathering devices with traceable sensors measuring the same light source (the sun) at the same instant in time, comparing different outdoor accelerated weathering devices to one another with respect to continuous direct measurement of each such device in a collection relating the quality of each single device back to a single standard measured by all devices and using a single spectrum measurement device design to rationalize the results of different methods of weatherability testing.
Conventional radiation measurements for outdoor weathering testing exposures are described in the appropriate primary standards: (1) ASTM G 90 describes the current method for outdoor accelerated weathering devices; and (2) ASTM G 7 describes the current method for outdoor real time weathering tests. Numerous other testing standard methodologies are known to those of skill in the art for specific applications and to attempt to list all such standards is unnecessary. In general, each conventional method uses broadband total solar and ultraviolet region measurement in calculations to determine the broadband solar radiant exposure of test specimens. These methods do not allow specific spectral wavelengths to be monitored for outdoor accelerated tests. These methods also do not allow measurement of radiant exposure of individual machines or a traceable method to determine the radiant exposure or spectral dose on individual outdoor accelerated weathering devices.
There are prior art references that disclose spectral monitoring devices. However, none of such references disclose methods for directly comparing indoor, outdoor real-time and outdoor accelerated methods. Nor do such references disclose calibration techniques or comparison of natural sunlight to concentrated natural sun light using the spectral monitoring devices of such references.
Generally, a variety of conventional methods have been used to determine radiant exposure (light dose) of specimens on natural end use non-accelerated exposures, namely: days, months and years by calendar; sunlight hours using focusing lenses and burned strips of paper; pyranometer devices measuring total solar radiation; and total ultraviolet radiometers (“TUVR”) devices measuring ultraviolet wavebands. Additionally, a variety of conventional methods have been used to determine radiant exposure in artificial weathering chambers; namely: time, broadband wavelength filtered detectors; devices, such as broadband and narrowband photodetectors and radiometers; and dose calculations based on radiation flux integrated overtime.
Still further, a few conventional methods have been used to determine radiant exposure in outdoor accelerated (concentrated) weathering methods, namely: radiometers mounted remote to the accelerated weathering devices; comparison of shaded disc radiometers to radiometers not shaded; radiometers fitted with collimating tubes; and calculations based on assumed reflectance values of concentrating elements.
Calibration methods used to calibrate solar radiation sensors for use in outdoor accelerated weathering methods and outdoor real-time weathering tests use a multi-step calibration method to link radiation measurements back to a primary reference standard. This calibration chain is incomplete with respect to tests performed in outdoor accelerated weathering methods such as the fresnel-reflecting solar concentration devices.
The conventional method of calibration is generally summarized as follows: a primary reference sensor at the exposure laboratory is calibrated to a traceable light source in the laboratory, which is a standard from any suitable standards organization, such as may be available from National Institute of Standards and Technology. For example, a spectrophotometer measures a standard light source and the signal at each of the wavelengths measured by the spectrophotometer is adjusted to match the standard specified values for that light source. This primary reference sensor is then used side by side with other master standard measurement devices to measure the sun at the same moment in time. For example, a standard-traceable spectrophotometer is set up outside, under the sun next to master standard TUVRs, such as Model TUVR from The Eppley Laboratory Inc. of Newport, R.I. Both the spectrophotometer and TUVR devices measure the sun at the same moment and the TUVRs are adjusted until they measure the same as the standard-traceable spectrophotometer primary sensor. Note that a spectrophotometer is a spectral instrument that measures light at discreet wavelengths but that the TUVR is a total ultraviolet instrument and measures all light between 295 nm and 385 nm wavelengths. Simple calculations allow spectral data to be integrated to total ultraviolet data.
The master standard TUVR instruments are subsequently mounted side-by-side with working TUVR instruments to measure the sun at the same moment and the working TUVRs are adjusted to read the same values as the master TUVRs.
Two calibrated working TUVRs are then mounted on a tracking mount that follows the sun's movements in a similar manner as conventional outdoor accelerated weathering devices—one under a shading disc and the other direct to the sun—and measurements are taken in accordance with ASTM G 90 to calculate the discreet component of solar flux reaching the laboratory and the accelerated weathering devices therein.
The conventional method of using an average mirror reflectance of the entire field collection of outdoor accelerated weathering devices (i.e., the average reflectance of the entire field collection of outdoor accelerated weathering devices multiplied by the number of mirrors on each device) generates a single average calculated dose over a broad wavelength region that is determined for all materials on exposure regardless of which individual machine characteristics those materials were exposed under. For a more thorough discussion refer to ASTM G 90, which is hereby incorporated by reference herein.
Conventional methods do not directly measure concentrated sunlight on outdoor accelerated weathering machines and do not directly compare actual sunlight to concentrated sunlight on outdoor accelerated weathering machines. Conventional methods also introduce error due to the broadband measurements that were taken of the source of sunlight and then inferred through calculation to be what was deposited after concentration on the outdoor accelerated weathering device target boards. In fact, the mirror spectral reflectance may have changed its specific reflectance characteristics, which would not have been observed by remotely located broadband detectors. The spectral changes would have gone unaccounted for in conventional methods.
Conventional methods also use reflective factors from single narrow wavelength reflectance measurements of reflective elements installed on the outdoor accelerated weathering devices. The narrow waveband reflectance measurements do not accurately describe the actual spectral reflectance of the reflective elements and can cause considerable errors in calculating the concentration factor of individual outdoor accelerating weathering devices.
In conventional methods, the accumulated radiant exposure is not measured by individual systems mounted on individual machines, rather it is measured by a common system and applied to a collection of unique machines ignoring machine-to-machine variations. Moreover, conventional methods are based on a mirror reflectance value on the fresnel-reflecting concentrating devices. This is typically an average value obtained by averaging measurements from many different individual machines and mirror elements within each machine. The actual individual values for specific machines vary greatly from the average.
Conventional methods also do not take into account characteristics of individual machines. For instance, the mirror alignment (accurate alignment of reflected light beams in the target area) differ in quality from machine to machine. These individual machine differences are not accounted for in conventional methods. By actually measuring the solar spectral power distribution on the target board of individual machines using the unique methods disclosed herein, a more accurate measurement of actual light flux and radiant exposure can be obtained.
Conventional methods to correlate results obtained on different outdoor accelerated weathering devices, use broadband total solar and ultraviolet region measurement in calculations to determine the solar radiant exposure of test specimens. These methods do not allow specific spectral wavelengths to be monitored for the test. Further, conventional methods do not replicate the measurements obtained in the xenon arc chamber tests on tests performed in outdoor natural real-time exposures or outdoor accelerated exposure tests. The unique methods disclosed herein repeat the same measurements using the same devices in all three exposure types. By using the same device designs, more comparable simulations can be made and results from the three different weathering test methods are more comparable than the results obtained from conventional methods. It is important to note that conventional outdoor accelerated methods use a broad spectral region and not spectral wavelengths unique to different individual materials
Therefore, there exists a need in the art for unique methods that allow true direct comparison of natural sunlight to concentrated sunlight in the target area of outdoor accelerated weathering devices, a more direct traceable path due to fewer steps in the calibration sequence for light monitors, all outdoor weathering devices in a field to be measured simultaneously using the same light source and correlation of weathering test results regardless of the type of weathering device used, that overcome the disadvantages described above, but also decrease cost and provide improved performance in use.